Many types of input devices are presently available for performing operations in a computing system, such as buttons or keys, mice, trackballs, touch sensor panels, joysticks, touch screens and the like. Touch screens, in particular, are becoming increasingly popular because of their ease and versatility of operation as well as their declining price. Touch screens can include a touch sensor panel, which can be a clear panel with a touch-sensitive surface, and a display device that can be positioned behind the panel so that the touch-sensitive surface can substantially cover the viewable area of the display device. Touch screens can allow a user to perform various functions by touching the touch sensor panel using a finger, stylus or other object at a location dictated by a user interface (UI) being displayed by the display device. In general, touch screens can recognize a touch event and the position of the touch event on the touch sensor panel, and the computing system can then interpret the touch event in accordance with the display appearing at the time of the touch event, and thereafter can perform one or more actions based on the touch event.
Touch sensor panels can be formed from a matrix of row and column traces, with sensors or pixels present where the rows and columns cross over each other while being separated by a dielectric material. Touch sensors can also be arranged in any number of dimensions and orientations, including diagonal, concentric circle, and three-dimensional and random orientations. In order to scan a touch sensor panel driven by multiple stimulation frequencies and compute an image of touch, various phases of selected low noise frequencies can be used to simultaneously stimulate the rows of the touch sensor panel, and multiple mixers can be configured to demodulate the signals received from the columns using the selected low noise frequencies. The demodulated signals from the multiple mixers, representing touch output values, can be used in calculations to determine an image of touch for the touch sensor panel at each frequency.
Touch sensor panels can be capable of detecting either single-touch events or multiple touch events, which are described in Applicant's co-pending U.S. application Ser. No. 11/649,998 entitled “Proximity and Multi-Touch Sensor Detection and Demodulation,” filed on Jan. 3, 2007, the contents of which are incorporated by reference herein.
In multi-touch sensor panels, certain pixels can generate false, erroneous or otherwise distorted readings when two or more simultaneous touch events are generated by the same poorly grounded object.